In the related art, there is known an OFDR device that measures a strain distribution or a temperature distribution of an optical fiber by using optical frequency domain reflectometry (OFDR) (for example, refer to Patent Document 1). The strain distribution and the temperature distribution of an optical fiber can be measured in the same manner. Thus, hereinafter, a configuration and operation of the OFDR device will be described in strain measurement as an example.
FIG. 15 illustrates a basic configuration of the OFDR device of the related art. A swept light source 41 sweeps the wavelength of output light such that the frequency of light linearly changes with respect to time. An optical coupler 42 causes the output light of the swept light source 41 to branch into two. One branch of the light branching at the optical coupler 42 is incident on a measured optical fiber 43, and the other is incident on a reference light optical fiber 44.
The measured optical fiber 43 includes fiber Bragg gratings (FBG) 431, 432, and 433 of a predetermined length. The FBGs may be arranged in plural quantities discretely as in FIG. 15, or there may be only one FBG.
The FBGs, when being arranged in plural quantities, may have different grating periods or may have the same grating period. The FBGs reflect light of a specific wavelength corresponding to the grating period thereof. The wavelength of the reflected light changes if a longitudinal strain is exerted on the measured optical fiber 43.
A reflecting coat (reflecting face) 44a that reflects light of all wavelengths is attached to the tip end of the reference light optical fiber 44. Light reflected by the measured optical fiber 43 and light (reference light) reflected by the reflecting face 44a of the reference light optical fiber 44 are combined by the optical coupler 42, and the reflected light from the measured optical fiber 43 interferes with the reference light. An optical receiver 46 outputs an electrical signal in proportion to the intensity of input light and outputs a beat signal from the interference of the reference light and the reflected light from the measured optical fiber 43. The electrical signal is converted into a digital signal by an A/D converter 47.
A spectrogram calculator 48 performs discrete Fourier-transform on the digital signal in each differential wavelength interval and calculates a spectrogram. A peak wavelength detector 49 detects the peak of the spectrogram in the direction of a wavelength axis in each differential frequency interval and outputs a strain at each position on the measured optical fiber 43. A feature of the present method is that a strain distribution in an FBG of a predetermined length can be measured since the strain is obtained in each differential frequency interval.
An FBG is configured of a grating that is formed by periodically changing a refractive index in the longitudinal direction of the optical fiber. Given that the grating period of the FBG is Λ, a reflected wavelength λ0 when there is no strain is represented as follows.λ0=2nΛ  (1)The symbol n is the refractive index of the measured optical fiber. A reflected wavelength change Δλ when a strain ε is exerted on the measured optical fiber in the longitudinal direction thereof is represented as follows.λ0+Δλ=2(n+Δn)(Λ+ΔΛ)  (2)
The symbol Δn is a refractive index change when the strain ε is exerted, and the symbol ΔΛ is a grating period change when the strain ε is exerted. Generally, Δn and ΔΛ are sufficiently smaller than n and Λ respectively. Thus, the following expression can be achieved.
                              Δλ                      λ            ⁢                                                  ⁢            0                          ≃                                            Δ              ⁢                                                          ⁢              n                        n                    +                      ΔΛ            Λ                          ≃                                            Δ              ⁢                                                          ⁢              n                        n                    +          ɛ                                    (        3        )            In a case of n=1.45 and λ0=1550 nm, Δλ≈1.2×10−6·ε is satisfied, and measuring the reflected wavelength change Δλ allows the strain ε of the measured optical fiber to be obtained.
The measured optical fiber 43 is assumed to have three reflection points of a point a, a point b, and a point c as illustrated in FIG. 16(a), and the distances thereof from a near end point o of the measured optical fiber 43 are designated by reference signs La, Lb, and Lc. If the distance of light from the optical coupler 42 reflected at the near end point o of the measured optical fiber 43 and returning to the optical coupler 42 is the same as the distance of the reference light from the optical coupler 42 reflected by the reflecting face 44a of the reference light optical fiber 44 and returning to the optical coupler 42, light reflected at the point a of the measured optical fiber 43 is combined by the optical coupler 42 with a time delay of ta=2nLa/c in comparison with the reference light. In the time delay, the symbol n is the refractive index of the measured optical fiber 43, and the symbol c is the speed of light.
Similarly, light reflected at the point b and light reflected at the point c are respectively delayed in time by tb=2nLb/c and tc=2nLc/c. An optical frequency νr of the reference light, an optical frequency νa of the reflected light from the point a, an optical frequency νb of the reflected light from the point b, and an optical frequency νc of the reflected light from the point c are illustrated in FIG. 16(b).
Given that an optical frequency change per unit time in the output light of the swept light source 41 is S, a beat frequency from the interference of the reference light and the reflected light from the point a is represented as follows.
                              f          a                =                                                                        v                a                            -                              v                r                                                          =                                    S              ·                              t                a                                      =                                                            2                  ⁢                  nS                                c                            ⁢                              L                a                                                                        (        4        )            Similarly, beat frequencies from the interference of the reference light with the reflected light from the point b and with the reflected light from the point c are represented as follows.
                              f          b                =                                                                        v                b                            -                              v                r                                                          =                                    S              ·                              t                b                                      =                                                            2                  ⁢                  nS                                c                            ⁢                              L                b                                                                        (        5        )                                          f          c                =                                                                        v                c                            -                              v                r                                                          =                                    S              ·                              t                c                                      =                                                            2                  ⁢                  nS                                c                            ⁢                              L                c                                                                        (        6        )            Accordingly, if a received signal is Fourier-transformed, beat signals of frequencies fa, fb, and fc are observed in proportion to the distances La, Lb, and Lc as in FIG. 16(c). A reflectance is assumed to be sufficiently small at each point, and multiple reflections are ignored. As described heretofore, a longitudinal distribution of reflection from the measured optical fiber can be measured by optical frequency domain reflectometry.
The spectrogram is obtained by performing short-time Fourier transform for each predetermined amount of time and represents intensities of light with color and shade on a two-dimensional plane formed by a time axis and a frequency axis. In each discrete Fourier transform, a window function is applied if necessary to make an overlap generally in a time domain. Since the output light of the swept light source 41 is wavelength-swept, the time axis is convertible into wavelength, and wavelength is convertible into strain from the above relationship. In addition, since the beat frequency corresponds to a distance on the measured optical fiber 43 as described above, the frequency axis is convertible into distance.
Accordingly, intensity data is obtained on a two-dimensional plane having a horizontal axis as distance and a vertical axis as strain. The spectrogram of a beat signal when a strain is not exerted on the measured optical fiber 43 is illustrated in FIG. 17(a). In FIG. 17, black represents low intensity, and white represents high intensity. The spectrogram, for example, if a strain is exerted on the measured optical fiber 43 is illustrated in FIG. 17(b). If the peak of the spectrogram is obtained in the direction of the wavelength axis, a strain distribution in the FBG of the measured optical fiber 43 is obtained as in FIG. 17(c) when a strain is not exerted and as in FIG. 17(d), for example, when a strain is exerted.
A relationship between the strain of the measured optical fiber 43 and the peak wavelength of the reflected light is illustrated in FIG. 18. Given that a relationship between the reflected wavelength change Δλ and the strain ε is Δλ=a·ε and that a measured wavelength range is Δλm, a strain measurable range Δε is represented as follows.
                    Δɛ        =                              Δλ            ⁢                                                  ⁢            m                    a                                    (        7        )            For example, in a case of a=1.2×10−6, a strain range of Δε=0.08% can be measured within a measured wavelength range of Δλm=1 nm.